Several alloy systems rely upon intermetallic precipitates for strengthening of mechanical properties. Intermetallics are especially useful for strengthening alloys at elevated temperatures. Typically, intermetallics are initially formed during solidification and cooling of an alloy. Homogenization and precipitation heat treatments are then used to control the size and distribution of the intermetallic precipitates. When the intermetallic precipitates are insoluble in the matrix, the size and distribution of the precipitates are extremely difficult to control.
As a consequence of relatively low melting temperatures, the strength of zinc and zinc-base alloys drops significantly with relatively small increases in temperature. For example, creep strength as well as tensile and yield strengths at 100.degree. C. are typically reduced to between 65 to 75% of the room temperature strengths (ASM Metals Handbook, 10th Edition, Volume 2, p. 529). Primary creep resistance is generally improved by reducing the volume of primary phase (near eutectic compositions) and by alloying with elements such as copper. Of the commercially available zinc alloys, ZA-8 (UNS Z35636 8-8.8 Al, 0.8-1.3 Cu, 0.015-0.030 Mg, 0.004 max. Cd, 0.06 max. Fe, 0.005 max. Pb, 0.003 max. Sn and balance Zn) has the highest primary creep resistance (in the Zn--Al binary, the eutectic composition is at 6% Al). This effect has been noted for zinc-aluminum alloys containing high volume fractions (30 vol %) of ceramic particles where a near order of magnitude difference in creep rate has been observed for ZA-8 alloy. (Tang et al, "Creep Testing of Pressure Die Cast ZA-8/TiB.sub.2, Composites," Advances in Science, Technology and Applications of Zn--Al Alloys, edited by Villasenor et al, 1994.)
Improvements in creep resistance of zinc alloys has been attained by processing techniques that reduce the size of primary dendrites or by addition of a ceramic dispersion phase. Dendrite size has been reduced by increasing the rate of solidification or by mixing the alloy in the semi-solid state (rheocasting). Rheocasting of semi-solid metals was developed in the 1970s by M. Flemings and R. Mehrabian. Examples of rheocasting are illustrated in U.S. Pat. Nos. 3,902,544 and 3,936,298. Rheocasting involves agitation of partially solidified metals to break up dendrites and form a solid-liquid mush. This solid-liquid mush is thixotropic in Theological behavior which allows casting of high solid volume fractions by injection molding and die casting. The above developers of rheocasting, as well as others, have also proposed incorporating ceramic particles into thixotropic semi-solid metals (Mehrabian et al, "Preparation and Casting of Metal-Particulate Non-Metal Composites", Met. Trans. A, Vol. 5, (1974) pp. 1899-1905). A disadvantage of this technique is that the metal/ceramic system may be chemically unstable. Ceramic particles may react with the metal matrix to degrade the reinforcing phase and form undesirable brittle phases at the particle/matrix interface. A further disadvantage of ceramic addition is that the choice of a suitable reinforcement is also subject to mixing problems associated with density differences or wetting phenomena. Particles such as certain borides or carbides may also be cost prohibitive in relation to the cost of the matrix metal.
The morphologies of materials cast by the above rheocasting processes are typically characterized by primary dendrites with diameters between 100 and 400 microns for Zn--10Cu--2Sn, 304 stainless steel and Sn--Pb alloys. Finer particle sizes on the order of 35 to 70 .mu.m were reported for ZA-27 alloy (UNS Z35841 25.0-28.0 Al, 2.0-2.5 Cu, 0.010-0.020 Mg, 0.004 max. Cd, 0.06 max. Fe, 0.005 max. Pb, 0.003 max. Sn and balance Zn) by Lehuy, Masounave and Blain ("Rheological behavior and microstructure of stir-casting zinc-aluminum alloys", J. Mat. Sci., 20 (1985), pp. 105-113). According to Lehuy et al, clustering occurred for volume fractions of solids that exceeded 35 percent and particle size distribution tended to decrease with increasing melt temperatures.
It is an object of the invention to provide a method to control the size and distribution of insoluble metal phases.
It is a further object of the invention to provide a method for producing zinc-base alloys containing insoluble metal phases via a semi-solid route or "mush casting".
It is a further object of the invention to provide magnesium-base and zinc-base alloys with improved creep strength at elevated temperatures.
It is a further object of the invention to provide a method for producing stable magnesium-base and zinc-base alloys to facilitate extended holding and solidification times.